1. Field of the Invention
The present invention relates to optical fiber transmission systems, more specifically to wavelength division multiplex transmission systems.
2. Description of the Prior Art
The index profile of optical fibers is generally characterized as a function of the shape of the graph of the function which associates the refractive index and the radius of the fiber. It is conventional to plot the distance r from the center of the fiber on the abscissa axis and the difference between the refractive index and the refractive index of the cladding of the fiber on the ordinate axis. The expressions xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d and xe2x80x9ctrianglexe2x80x9d are used for the index profiles of graphs which are respectively step-shaped, trapezium-shaped and triangular. These curves are generally representative of the theoretical or set point profile of the fiber and fiber fabrication constraints can yield a significantly different profile.
It is advantageous to control chromatic dispersion in new wavelength division multiplex transmission networks using high bit rates, especially bit rates greater than or equal to 40 Gbit/s or 160 Gbit/s; the objective is to obtain substantially zero cumulative chromatic dispersion over the link for all wavelengths of the multiplex, in order to limit widening of the pulses. A cumulative dispersion value of several tens of ps/nm is generally acceptable. It is also useful to avoid zero values of local chromatic dispersion, at which nonlinear effects are more accentuated, in the vicinity of wavelengths used in the system. Finally, it is also useful to limit the chromatic dispersion slope over the range of the multiplex; low slope values limit distortion between channels of the multiplex and facilitate compensation of chromatic dispersion over the whole of the multiplex. The chromatic dispersion slope is conventionally the derivative of the chromatic dispersion with respect to wavelength.
Stepped-index fiber, also known as single-mode fiber (SMF), is conventionally used as line fiber in optical fiber transmission systems. The prior art ASMF 200 single-mode fiber has a chromatic dispersion of the order of 17 ps/(nm.km) at 1 550 nm. The chromatic dispersion slope at 1 550 nm is of the order of 0.06 ps/(nm2.km).
Dispersion-shifted fibers (DSF) are now available off the shelf. Non-zero dispersion-shifted fibers (NZ-DSF+) are dispersion-shifted fibers with a positive non-zero chromatic dispersion at the wavelengths at which typically around 1 550 nm. At these wavelengths these fibers have a low chromatic dispersion, typically less than 11 ps/(nm.km) at 1 550 nm, and a chromatic dispersion slope from 0.04 ps/(nm2.km) to 0.1 ps/(nm2.km).
FR-A2 790 107 proposes a line fiber which is particularly suitable for dense wavelength division multiplex transmission with a channel spacing of 100 GHz or less for a bit rate per channel of 10 Gbit/s or more; at a wavelength of 1 550 nm, this fiber has an effective surface area greater than or equal to 60 xcexcm2, a chromatic dispersion from 6 ps/(nm.km) to 10 ps/(nm.km), and a chromatic dispersion slope less than 0.07 ps/(nm2.km). At this wavelength, the bending losses with the fiber wound around a 10 mm radius former are of the order of 10 dB/m. At a wavelength of 1 625 nm, the bending losses reach 50 dB/m.
French patent application No. 0002316 filed Feb. 24, 2000 (published Aug. 31, 2001, publication number 2 805 620), whose title in translation is xe2x80x9cAn optical fiber providing monomode propagation when incorporated into a cable, for wavelength division multiplex optical fiber transmission networksxe2x80x9d, proposes a line fiber having at 1 550 nm a chromatic dispersion from 5 ps/(nm.km) to 11 ps/(nm.km), a chromatic dispersion slope from 0.014 ps/(nm2.km) to 0.044 ps/(nm2.km) and bending losses measured by winding the fiber around a 10 mm radius former of the order of 10 dB/m. At a wavelength of 1 625 nm the bending losses reach 50 dB/m.
The invention proposes a fiber which has a low (virtually zero) chromatic dispersion slope compared to these non-zero dispersion-shifted fibers and thus an almost constant chromatic dispersion over a wider range of wavelengths, which limits distortion between channels. The fiber can therefore be used in a wavelength division multiplex transmission system with a larger number of channels.
To be more precise, the invention proposes an optical fiber in which propagation is monomode propagation at a range of wavelengths from 1 300 nm to 1 700 nm when incorporated into a cable, said fiber having:
at a wavelength of 1 550 nm, a chromatic dispersion C from 3 ps/(nm.km) to 14 ps/(nm.km) and an absolute value of chromatic dispersion slope Cxe2x80x2 less than 0.014 ps/(nm2.km), and
at a wavelength of 1 625 nm, bending losses measured for a 10 mm radius less than 400 dB/m.
The fiber can advantageously have one or more of the following optical characteristics:
at a wavelength of 1 550 nm an effective surface area greater than or equal to 35 xcexcm2, preferably greater than or equal to 40 xcexcm2;
a chromatic dispersion cancellation wavelength xcex0 less than or equal to 1 400 nm;
at a wavelength of 1 550 nm a mode diameter 2W02 greater than or equal to 6.4 xcexcm;
at a wavelength of 1 550 nm bending losses measured for a 10 mm radius less than 100 dB/m;
at a wavelength of 1 625 nm, and preferably at a wavelength of 1 675 nm, bending losses of less than 400 dB/m measured for a 10 mm radius;
at a wavelength of 1 550 nm bending losses of less than 10xe2x88x922 dB measured for 100 turns of fiber around a 30 mm radius former;
at a wavelength of 1 625 nm bending losses of less than 0.1 dB measured for 100 turns of fiber around a 30 mm radius former;
at a wavelength of 1 675 nm bending losses of less than 0.5 dB measured for 100 turns of fiber around a 30 mm radius former;
a theoretical cut-off wavelength less than or equal to 1 850 nm and preferably less than or equal to 1 800 nm;
an absolute value of the variation of chromatic dispersion at wavelengths from 1 460 nm to 1 625 nm, relative to its value at 1 550 nm, less than or equal to 35%, and preferably less than or equal to 25%, or even less than or equal to 20%;
an absolute value of the variation of chromatic dispersion at wavelengths from 1 460 nm to 1 675 nm, relative to its value at 1 550 nm, less than or equal to 40%, and preferably less than or equal to 35%, or even less than or equal to 25%;
an absolute value of the variation of chromatic dispersion at wavelengths from 1 460 nm to 1 625 nm, relative to its value at 1 550 nm, less than or equal to 2 ps/(nm.km), and preferably less than or equal to 1 ps/(nm.km);
an absolute value of the variation of chromatic dispersion at wavelengths from 1 460 nm to 1 675 nm, relative to its value at 1 550 nm, less than or equal to 3 ps/(nm.km), and preferably less than or equal to 2 ps/(nm.km);
at a wavelength of 1 550 nm a chromatic dispersion C from 5 ps/(nm.km) to 11 ps/(nm.km);
at a wavelength of 1 550 nm an absolute value of the chromatic dispersion slope Cxe2x80x2 less than or equal to 0.012 ps/(nm2.km) and preferably less than 0.010 ps/(nm2.km);
an absolute value at a wavelength of 1 550 nm of the product of the effective surface area and the ratio between chromatic dispersion and chromatic dispersion slope greater than 12 000 xcexcm2.nm;
at a wavelength of 1 550 nm a sensitivity to microbending less than or equal to 1 and preferably less than or equal to 0.8;
at a wavelength of 1 550 nm an attenuation less than or equal to 0.24 dB/km;
at a wavelength of 1 550 nm a polarization mode dispersion less than or equal to 0.2 ps/km1/2, and preferably less than or equal to 0.1 ps/km1/2, or even less than or equal to 0.05 ps/km1/2.
The index profile of one embodiment of the fiber is a trapezium or rectangle with a buried trench and a ring. In this case, the characteristics of the profile can advantageously be chosen from the following characteristics:
the difference (xcex94n1) between the index of the trapezium or the rectangle and the index of the cladding is from 5.7xc3x9710xe2x88x923 to 9xc3x9710xe2x88x923 and the radius (r1) of the portion of the fiber having an index greater than that of the cladding is from 2.8 xcexcm to 4.2 xcexcm;
the difference (xcex94n2) between the index of the depleted trench and the index of the cladding is from xe2x88x929xc3x9710xe2x88x923 to xe2x88x922.5xc3x9710xe2x88x923 and the outside radius (r2) of said trench is from 5.6 xcexcm to 7.9 xcexcm;
the difference (xcex94n3) between the index of the ring and the index of the cladding is from 1xc3x9710xe2x88x923 to 5.7xc3x9710xe2x88x923 and the outside radius (r3) of said ring is from 7.8 xcexcm to 10.5 xcexcm;
the ratio between the inside radius and the outside radius of the trapezium is from 0.55 to 1;
the integral of the index between a zero radius and the outside radius (r1) of the central portion of the fiber having an index greater than that of the cladding is greater than 23xc3x9710xe2x88x923 xcexcm;
twice the integral of the product of the index and the radius between a zero radius and the outside radius (r1) of the central portion of the fiber having an index greater than that of the cladding is from 65xc3x9710xe2x88x923 xcexcm2 to 90xc3x9710xe2x88x923 xcexcm2;
three times the integral of the product of the index and the square of the radius between a zero radius and the outside radius (r1) of the central portion of the fiber having an index greater than that of the cladding is from 180xc3x9710xe2x88x923 xcexcm3 to 305xc3x9710xe2x88x923 xcexcm3;
the integral of the index between the outside radius (r1) of the central portion of the fiber having an index greater than that of the cladding and the inside radius (r2) of the ring is from xe2x88x9227xc3x9710xe2x88x923 xcexcm to xe2x88x9212xc3x9710xe2x88x923 xcexcm;
the integral of the index between the inside radius (r2) and the outside radius (r3) of the ring is from 4xc3x9710xe2x88x923 xcexcm to 12xc3x9710xe2x88x923 xcexcm.
In another embodiment the index profile of the fiber is a rectangle with a first buried trench, a ring, and a second buried trench. The characteristics of this profile can advantageously be chosen from the following characteristics:
the difference (xcex94n1) between the index of the rectangle and the index of the cladding is from 6xc3x9710xe2x88x923 to 9xc3x9710xe2x88x923 and the outside radius (r1) of the rectangle is from 2.3 xcexcm to 4.1 xcexcm;
the difference (xcex94n2) between the index of the first depleted trench and the index of the cladding is from xe2x88x929xc3x971xe2x88x923 to xe2x88x922.5xc3x9710xe2x88x923 and the outside radius (r2) of said trench is from 4.8 xcexcm to 8.8 xcexcm;
the difference (xcex94n3) between the index of the ring and the index of the cladding is from 1xc3x9710xe2x88x923 to 6.5xc3x9710xe2x88x923 and the outside radius (r3) of said ring is from 9.5 xcexcm to 14.2 xcexcm;
the difference (xcex94n4) between the index of the second depleted trench and the index of the cladding is from xe2x88x928xc3x9710xe2x88x923 to xe2x88x922xc3x9710xe2x88x923 and the outside radius (r4) of said trench is from 12.6 xcexcm to 17 xcexcm;
the integral of the index between a zero radius and the outside radius (r1) of the rectangle is from 25xc3x9710xe2x88x923 xcexcm to 30.5xc3x9710xe2x88x923 xcexcm;
twice the integral of the product of the index and the radius between a zero radius and the outside radius (r1) of the rectangle is from 75xc3x9710xe2x88x923 xcexcm2 to 105xc3x9710xe2x88x923 xcexcm2;
three times the integral of the product of the index and the square of the radius between a zero radius and the outside radius (r1) of the rectangle is less than 380xc3x9710xe2x88x923 xcexcm3;
the integral of the index between the outside radius (r1) of the rectangle and the inside radius (r2) of the ring is from xe2x88x9231.5xc3x9710xe2x88x923 xcexcm to xe2x88x9213.5xc3x9710xe2x88x923 xcexcm;
the integral of the index between the inside radius (r2) and the outside radius (r3) of the ring is from 6xc3x9710xe2x88x923 xcexcm to 21xc3x9710xe2x88x923 xcexcm;
the integral of the index between the inside radius (r2) of the ring and the outside radius (r4) of the second buried trench is greater than xe2x88x9222xc3x9710xe2x88x923 xcexcm;
twice the integral of the product of the index and the radius between the outside radius (r1) of the rectangle and the outside radius (r3) of the ring is from xe2x88x92135xc3x9710xe2x88x923 xcexcm2 to 245xc3x9710xe2x88x923 xcexcm2;
twice the integral of the product of the index and the radius between the inside radius (r2) of the ring and the outside radius (r4) of the second buried portion is less than 165xc3x9710xe2x88x923 xcexcm2.
The invention further proposes a wavelength division multiplex transmission system comprising a line fiber consisting of the above kind of fiber.
It is advantageous if the system further comprises a line fiber having at a wavelength of 1 550 nm a positive non-zero chromatic dispersion and a ratio of chromatic dispersion to chromatic dispersion slope from 50 nm to 400 nm. In this case, the length of the fiber according to the invention is preferably greater than or equal to the length of the fiber having a ratio of chromatic dispersion to chromatic dispersion slope from 50 nm to 400 nm.
There can also be provided a dispersion-compensating fiber having at a wavelength of 1 550 nm a ratio of chromatic dispersion to chromatic dispersion slope greater than or equal to 250 nm.
The transmission system advantageously has a range of use that comprises wavelengths from 1 460 nm to 1 625 nm, and preferably wavelengths from 1 460 nm to 1 675 nm, and even wavelengths from 1 300 nm to 1 700 nm.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which description is given by way of example only and with reference to the accompanying drawings.